[KevinH]

But that is a epubcheck choice, their org, the idpf wrote the spec and designed the schema for an OPF, and a wrong one in my opinion since it violates the xml spec which came first and says that xml:lang is an always valid optional tag.

And there is no good reason for it. Most books are published in one language at a time and metadata in that same language is typical. So stating it once on the package tag makes the most sense. Assuming it is the same as the dc:language value is simply not a great idea. The OPF metadata could be in one language while the epub is actually in another, and knowing the xml:lang of the OPF itself is still needed imho.